Phenolic resin molded articles have widely been used alone or in combination with other resins such as epoxy resin, etc. for a long time because of excellent mechanical characteristics, but had a problem that the light-resistance and alkali resistance are slightly low, the dimension and electric resistance are liable to change by absorption of water or an alcohol, and the heat resistance (particularly oxidation resistance at high temperature) is inferior.
Therefore, as a method of solving such a problem, various modifications of the phenolic resin have been studied. For example, there have been suggested a lot of modified phenolic resins wherein the resistance to deterioration or oxidation due to light, chemical, etc. is improved by modification using fats, oils, rosin or a neutral aromatic compound.
For example, Japanese Patent Laid-Open Publication No. 61(1986)-235413 discloses that a phenolic resin having excellent heat resistance can be obtained by selecting a reaction component of a phenol modified aromatic hydrocarbon resin. However, the phenolic resin obtained by this method had a drawback that the resin must be maintained at a high temperature for a long time when a molded article is produced by using the phenolic resin.
Japanese Patent Laid-Open Publication No. 2(1990)-27414 discloses that a modified phenolic resin useful for molding material, having excellent heat resistance, oxidation resistance and mechanical strength, as cannot be expected from a conventional phenolic resin, is obtained by employing petroleum heavy oils or pitches as a cheap raw material and selecting special reaction condition.
Further, Japanese Patent Laid-Open Publication No. 4(1992)-145116 discloses that, in the production of such a phenolic resin, a crude modified phenolic resin obtained by a polycondensation of starting compounds is subjected to a neutralization treatment, a water washing treatment and/or an extraction treatment to thereby neutralize and remove any acid remaining in the crude modified phenolic resin, so that a modified phenolic resin which does not corrode a metal member brought into contact with the resin is provided.
In the above process for producing the modified phenolic resin, the acid remaining in the crude modified phenolic resin is actually neutralized and removed by the neutralization treatment using an amine, followed by the water washing treatment. However, the modified phenolic resin obtained through the purification step involving the above neutralization and water washing treatments is likely to retain a neutralization product therein, so that there is a problem that it is unsatisfactory as a molding material used for a product on which strict requirements for thermal and corrosion resistance are imposed, such as a molding material for electrical or electronic part and a material for semiconductor sealer.
Japanese Patent Laid-Open Publication No. 6(1994)-228257 teaches that a modified phenolic resin containing substantially no acid can be obtained by purifying a crude modified phenolic resin through a purification step including a specific extraction treatment. The modified phenolic resin containing substantially no acid, obtained through this purification step, may be combined with an epoxy resin, so that a molding material can be obtained, which not only has excellent thermal and moisture resistance but also does not corrode any metals.
However, the above modified phenolic resin has a drawback in that the melt viscosity of the resin is so high that the resin is not suitable for speedy mass production of a molded article having a complex configuration. In addition, further improvements of thermal resistance, dimensional stability and strength and other mechanical properties have been demanded in the use of the modified phenolic resin in combination with an epoxy resin.
The present inventors proposed a process for producing a highly reactive modified phenolic resin having a low resin melt viscosity and an improved reactivity with the epoxy resins by means of reacting a modified phenolic resin with a phenol in the presence of an acid catalyst to thereby lower the molecular weight of the modified phenolic resin (See Japanese Patent Laid-Open Publication No. 7(1995)-252339 and Japanese Patent Application No. 8(1996)-24173).
The highly reactive modified phenolic resins obtained as described above are relatively low in viscosity and are capable of providing a molding material having good thermal resistance and moldability, as well as superior mechanical strength such as dimensional stability when combined with an epoxy resin.
However, according to such a method of producing the modified phenolic resin, two steps of the polycondensation step and the molecular-weight lowering step are required and, therefore, the production step became complicated.
By the way, in case of soldering which is generally performed on production of electrical appliances, electrical/electronic parts are exposed to high temperature. From this point of view, a further improvement in heat resistance is requested to the modified phenolic resin used as a resin material for electrical/electronic parts or semiconductor sealing material.
When the moisture absorption property of the resin material is high, there arises a problem that water is rapidly evaporated on soldering to cause a package crack and has a capability of corroding metal material combined with the resin.
The resin material is often used in combination with a metal member in the field of the electrical/electronic parts or semiconductor sealing material, and the reliability of the product is largely influenced by the adhesion between the resin material and metal member.
Accordingly, a further improvement in moisture absorption and that in adhesion were also rquired to the modified phenolic resin.
In addition to the simplification of the production step and improvement in characteristics of the resulting modified phenolic resin as described above, it is also highly requested to use a raw oil which can be obtained more effectively and stably and is advantageous in view of the cost, in place of a raw oil obtained by redistilling a residual oil obtained in a conventional catalytic cracking step.
The present inventors have studied intensively about a method of producing a highly reactive modified phenolic resin having excellent characteristics described above more simply. As a result, it has been found that, by mixing specific raw materials (including a catalyst) in a specific ratio to polycondense them, a highly reactive modified phenolic resin having high reactivity with the epoxy resin and low viscosity can be obtained in one step.
The present inventors also have found that a molding material prepared by using this highly reactive modified phenolic resin in combination with an epoxy resin can provide a molded article which is excellent in heat resistance, moisture resistance, corrosion resistance, adhesion, and mechanical characteristics (e.g. dimensional stability, strength, etc.), particularly moisture resistance and heat resistance.
The present invention has been accomplished based on these findings of the present inventors.